When is a
mouse like a test tube?
Robert Sanders, Media Relations
cells decorate their surfaces with a variety of sugars and
sugar polymers, called oligosaccharides. Carolyn Bertozzi
attaches unnatural chemicals to simple sugars and feeds
them to cells in order to get these chemicals onto the cell
surface as part of the sugary landscape. (Bertozzi
Bertozzi has put a new twist on the standard chemistry experiment:
Instead of using a test tube or flask, she mixes and reacts chemicals
in living organisms.
innovative approach involves chemicals that don't interact with
the molecules in the body, only with each other. But her in vivo
chemistry has great potential for studying cells in living organisms
and creating new diagnostics, and perhaps treatments, for disease.
using the mouse as a reaction vessel, designing chemical reactions
that will teach us about biology and disease without causing physiological
harm," said Bertozzi, a professor of chemistry and a faculty
scientist at Lawrence Berkeley National Laboratory. "It's
a really powerful technique, with the ability to change how we
think about applying chemical processes in biology."
and her colleagues report their novel chemical experiments - the
first reaction in a living organism between two chemicals that
don't react with the organism - in the Aug. 19 issue of Nature.
this particular experiment, they showed that they could use this
type of chemical reaction to tag cells in live mice, specifically,
to attach tracer molecules to sugars on the surface of cells.
The sugars Bertozzi targeted are produced abundantly by inflamed
cells and by cancer cells, which means her technique could be
used to attach medical tracers to such cells to allow doctors
to pinpoint them in the body.
fact that this works is remarkable," wrote David A. Tirrell
of the California Institute of Technology in an accompanying News
& Views article in Nature. "The labeling strategy described
by Bertozzi and colleagues allows one to probe the set of sugars
arrayed by the cell, to explore biosynthetic pathways and to examine
the functional consequences of modifying the complement of cell-surface
an investigator in the Howard Hughes Medical Institute at UC Berkeley
and a member of the California Institute for Quantitative Biomedical
Research (QB3), is a sugar chemist who studies the sugars and
sugar polymers (oligosaccharides and carbohydrates) with which
cells decorate themselves. Such sugars play major roles in cell-cell
interactions, and often provide entrée to bacteria and
viruses that cause disease.
than five years ago, she conceived of the idea of instigating
reactions in the body between unnatural chemicals - ones created
by humans and never before seen by living organisms. The unnatural
chemicals she chose do not react with any biological molecules,
so that the only reaction is between them, as if the body were
simply a flask of water.
idea runs counter to conventional techniques, where chemists try
to control the environment of a reaction by removing as many other
chemicals as possible.
chemists try to eliminate all interfering chemical groups, which
can be difficult in a biological system," Bertozzi said.
"We design functional groups to be specific, so they don't
react with the environment in the body. Ours is an elegant system,
highly targeted and very selective."
reaction she focused on was a simple one between azides and phosphines,
described last century by German synthetic-organic chemist Hermann
Staudinger, a pioneering polymer chemist who won the Nobel Prize
in 1953. Azides are simple, three-nitrogen molecules that can
be added to biological molecules, in particular sugars, without
really being noticed by the body. Phosphines are phosphorous-containing
molecules that react with azides to form a stable phosphorous-nitrogen
compound. Both types of molecules have shown no harmful effects
in the body, based on use of the AIDS drug AZT (azidothymidine)
and phosphine-gold compounds for treating arthritis.
figured that if she could attach an azide to a natural sugar and
feed it to cells, she could insinuate the sugar into the sugar
polymers (oligosaccharides) that decorate the exterior of cells,
then use the Staudinger ligation to attach phosphines to these
azides. If a label, such as a fluorescent dye or a contrast agent,
were attached to the phosphine, she would be able to label cells
for diagnostic purposes.
early cell culture experiments reported in 2000, Bertozzi showed
that an azide attached to the sugar mannose is taken up by cells
in a test tube and converted to another type of sugar, sialic
acid, which in turn is incorporated into sugar polymers on the
cell surface. More importantly, the more azide-sugar she fed cells,
the more appeared on the surface.
the current experiment, her research group extended this to living
mice. Graduate students Jennifer Prescher and Danielle
Dube injected the mice with an azide-mannose compound for
seven days, then harvested spleens from the mice to look for the
azide. As predicted, it appeared on the surface of spleen cells
attached to sialic acid, with more azide on the surface associated
with higher doses of the azide-mannose sugar. Azide-labeled cells
also were found in the heart, kidney and liver, though not the
brain, thymus or gut.
then repeated the experiment, but on the eighth day injected each
mouse with a phosphine attached to a fluorescent molecule called
FLAG. After only 90 minutes, the FLAG tag appeared on spleen cells,
proving that the Staudinger ligation had occurred in living mice.
successfully marked cells as a function of the robustness of a
metabolic pathway, the sialic acid biosynthetic pathway,"
Bertozzi said. "This offers an alternative way to visualize
cells that are undergoing a change in metabolism, such as happens
with cancer cells and inflamed cells that over-produce sialic
acid. The technique opens the door to non-invasive imaging of
sugars as markers of disease."
of its biological relevance, the method introduced by Bertozzi
and colleagues is remarkable as a chemical process," Tirrell
added in his commentary. "The fact that specific chemical
transformations can now be accomplished with spatial and temporal
control in live animals is a major step forward for chemistry."
and colleagues are now developing radio-labeled azide compounds
that could be detected by PET (positron emission spectroscopy)
or SPECT (single photon emission computed tomography), fluorescent
compounds that could be detected visually, and magnetic compounds
that could be detected by MRI (magnetic resonance imaging). She
also is investigating another type of probe that is not a phosphine
but will also react with azides in a highly selective manner.
work was supported by the Department of Energy.